Accurate clock generators or timing references have generally relied upon crystal oscillators, such as quartz oscillators, which provide a mechanical, resonant vibration at a particular frequency. The difficulty with such crystal oscillators is that they cannot be fabricated as part of the same integrated circuit (“IC”) that is to be driven by their clock signal. For example, microprocessors such as the Intel Pentium processor require a separate clock IC. As a consequence, virtually every circuit requiring an accurate clock signal requires an off-chip clock generator. Accordingly, accurate and stable clocking capability is one of the last functional components in electronic systems that have not been subject to integration.
There are several consequences for such non-integrated solutions. For example, because such a processor must be connected through outside circuitry (such as on a printed circuit board (PCB)), power dissipation is comparatively increased. In applications which rely on a finite power supply, such as battery power in mobile communications, such additional power dissipation is detrimental.
In addition, such non-integrated solutions, by requiring an additional IC, increase space and area requirements, whether on the PCB or within the finished product, which is also detrimental in mobile environments. Moreover, such additional components increase manufacturing and production costs, as an additional IC must be fabricated and assembled with the primary circuitry (such as a microprocessor).
Other clock generators which have been produced as integrated circuits with other circuits are generally not sufficiently accurate, particularly over fabrication process, voltage, and temperature (“PVT”) variations. For example, ring, relaxation and phase shift oscillators may provide a clock signal suitable for some low-sensitivity applications, but have been incapable of providing the higher accuracy required in more sophisticated electronics, such as in applications requiring significant processing capability or data communications. In addition, these clock generators or oscillators often exhibit considerable frequency drift, jitter, have a comparatively low Q-value, and are subject to other distortions from noise and other interference.
As a consequence, a need remains for a reference signal or clock generator which may be integrated monolithically with other circuitry, as a single IC, or which may be a discrete IC utilized to provide a clock or other reference signal to the other circuitry, and which is highly accurate over PVT variations. Such a reference signal or clock generator should be free-running and/or self-referencing, and should not require locking or referencing to another reference signal. Such a reference signal or clock generator should exhibit minimal frequency drift and have comparatively low jitter, and should be suitable for applications requiring a highly accurate system clock. Such a clock generator or timing reference should provide for control over output frequency, to provide a stable and desired frequency in response to variation in ambient or junction temperature or variation in other parameters such as voltage, fabrication process, frequency, and age. Lastly, such a clock generator or timing reference should also provide multiple operating modes, including a spread spectrum mode to reduce potential interference with other signals.